[Sidenote: General Collection of Minerals.]
In the table-cases of the gallery are exhibited specimens of every important Mineral species and variety preserved in the Museum. The cases containing this general collection are numbered from 1 to 41, and the eight panes of each case are severally distinguished by the letters _a_ to _h_. For the use of the student there is published an Index to the names of all the numerous Mineral species and varieties represented in the collection, with references to the table-cases in which the specimens are placed.[23]
The system of classification, which includes not only ores, but all known Minerals, is not easy for the visitor to follow, and it is therefore convenient to indicate the positions in the gallery of those minerals--as, for instance, precious and ornamental stones, and metallic ores--which have an interest for all; for details, reference must be made to the Mineral Guide. Most of these Minerals occur as crystals, the forms of which can be referred to six systems of crystallisation.
[Illustration: FIG. 58.--THE COLENSO DIAMOND, FROM SOUTH AFRICA. NATURAL SIZE.]
I. In cases 1 and 2 are the native metals, as Copper, Silver, Gold, and Platinum; and non-metals, as Sulphur, Diamond, and Graphite. The large symmetrical South African “Colenso” Diamond (fig. 58), weighing 130 carats, presented by the late Professor John Ruskin, is worthy of special attention (case 1f). Models of some famous diamonds, including “The Cullinan Diamond,” the largest ever found (weight before being cut, 3025¾ carats, or about 1⅓ lb. av.) are exhibited.
II. The next six cases contain Minerals which have mostly a metallic lustre and consist of metals in chemical combination with elements of the Sulphur or Arsenic groups.
Argentite (3d) is an important Silver-ore, containing 87 per cent. of Silver and 13 of Sulphur.
Blende (4b) is a valuable Zinc-ore, and contains 67 per cent. of Zinc and 33 per cent. of Sulphur.
Galena (4e) is by far the most important ore of Lead (Lead 87, Sulphur 13, per cent.).
Copper-glance (3e) is a common ore of Copper (Copper 80, Sulphur 20, per cent.).
Cinnabar (3h) is the ore from which Mercury or Quicksilver is obtained (Mercury 86, Sulphur 14, per cent.).